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Background and Objectives: Globally the survival of ALL has increased tremendously with the five-year overall survival (OS) reaching 90% in high-income countries (HICs) (1, 2). However, in India, the five-year OS has been reported to be 30-70 %( 3) and deaths due to treatment related toxicity (TRT) in ALL were ranged from 2-24 % (4), which is ten times higher compared to HICs suggesting increased susceptibility to the toxicity of the chemotherapeutic drugs. This could be due to frequent variants in candidate genes determining the pharmacodynamic response or pharmacokinetics of the chemotherapeutic drugs used during maintenance therapy of ALL. Variations in the patient management, supportive care therapy are the other possible reasons of this increased incidence of TRT. Many patients are lost to follow-up due to the TRT, and eventually will die of the disease progression. The present study aimed to explore the association of common genetic variants in the candidate genes with early treatment related hematological toxicities (grade 3-4) in patients with Acute Lymphoblastic Leukemia (ALL) receiving low-dose MTX (LDMTX) and 6-Mercaptopurine (6-MP) based maintenance therapy. Materials and Methods: This prospective study was conducted between August 2011 and May 2016 and approved by institutional scientific and ethics committees. A total of 71 patients (43 males and 28 females aged between 1-51 years) with ALL were enrolled in the study after obtaining written informed consent and in the case of children, from legally accepted guardians. Patients below 25 years of age were treated using protocol-841 (MCP) I2A and those older than 25 years were treated with modified GMALL-84 protocols. MTX and 6-MP doses did not differ across both the protocols during maintenance therapy. Germline DNA samples collected at the time of remission from peripheral mononuclear cells was used to genotype15 selected variants frequent in the following candidate genes : ABCB1, DHFR, GGH, FPGS, MTHFR, RFC1, SLCO1B1, TPMT, and NUDT15 using allele discrimination assay by real-time PCR. LC-MS/MS method was used to measure methotrexate polyglutamate (MTXPG3-5) levels in RBC's.Toxicities were graded according to the Common Terminology Criteria for Adverse Events (CTCAE-version 4.03). Early grade 3-4 hematological toxicities occurring within first 100 days of the maintenance therapy was studied for its association with the genetic variants and other risk factors. Cumulative incidence curves were plotted and a risk factor analysis was performed using multivariate Cox regression. Co-linearity between variables was assessed. We used a back-ward selection method, retaining those variables with P-values Results: The cumulative incidences of early hematological TRT (grade 3-4) and relapse were 54.9 %, and 38.0 %, respectively. The relapse free survival was 59.2 %. The median follow-up of all patients from the start of maintenance was 1018 days. In multivariate analysis including all genetic variants, age, and WBC counts at diagnosis, we observed a significant increase in the risk of TRT in carriers of NUDT15*3 allele(rs116855232 ; p=0.002 ; univariate Hazards ratio : 2.81 (95% CI : 1.41-5.59). 3435C>T variant in ABCB1 gene showed a trend of association with that of relapse free survival. Neither the genetic variants studied were associated with the methotrexate polyglutamate levels, nor were the levels associated with the clinical outcomes. Conclusion: The NUDT15*3 allele carriers status could be used as one of the stratifying markers in South Indian ALL patients at the time of diagnosis to distinguish high and low-risk patients to develop early hematological toxicity, especially related to 6-mercaptopurine based ALL maintenance therapy protocols. Disclosures No relevant conflicts of interest to declare.